In the early days of DBS (direct broadcast satellite) communications, satellite transponders were placed in widely spaced geosynchronous orbits, the earliest satellites transmitting in the "C-band" RF frequencies from 3.7 GHz to 4.2 GHz. Satellites launched more recently transmit either in the C-band or in the higher frequency "Ku" band from 11.7 GHz to 12.2 GHz.
Ground satellite receiving antennas have been constructed to simultaneously receive signals transmitted by a number of satellites in various frequency bands, including the C-band and Ku-band. One such ground satellite receiving antenna manufactured by Comsat/RSI (herein the "Torus" antenna) has a large toroidal reflector, of circular cross-section in longitude (azimuth) and of parabolic cross-section in elevation. It observes a seventy-degree sweep of the sky above the equator, and may receive transmissions from a multiplicity (e.g., more than thirty) of satellites in frequency bands including the C-band and the Ku-band.
The Torus antenna has an arcuate "bridge", offset from the central axis of the reflector by approximately 26 degrees, which supports a number of "feedhorns" (or simply "feeds") at locations along the bridge corresponding to the focal points of transmissions from the various satellites. The feedhorns detect and process the received satellite transmissions, as follows.
The RF energy from a particular satellite is collected by the antenna reflector and is focused to a narrow, approximately elliptical, zone of intense RF energy at the location of the feedhorn. In known systems, to maximize the strength of the signal received from a particular satellite, the feedhorn is first positioned along the bridge at a location indicated by a computer program provided by the antenna manufacturer. The angular elevation of the feedhorn is adjusted for peak signal strength, and its lateral position along the bridge is then fine-tuned. Finally, the angular elevation of the feedhorn is adjusted to peak the received signal.
At the feedhorn the focused RF signal energy from the selected satellite is collected and further focused by a conically shaped collector, detected by an RF probe, and amplified and block downconverted in an "LNB" (low-noise block converter).
Within the past few years, multiple satellites transmitting in different frequency bands have been placed in closely adjacent geosynchronous orbits. For example, two such "co-located" satellites, the "Galaxy 6" and the "SBS-6" satellites, are separated by only 0.05 degrees of longitude. The Galaxy 6 satellite is located at 74 degrees west longitude, transmitting in the C band, and the SBS-6 satellite is located at 74.05 degrees west longitude, transmitting in the Ku band.
Very recently "hybrid" or "double payload" satellites are being placed in orbit which broadcast signals in both the C band and the Ku band. Co-located and hybrid satellites effectively transmit signals from a common sky location. As standard receiving feedhorns are adapted to receive transmissions only in a single frequency band, the transmission of co-located signals creates a problem at the ground satellite receiving antenna.
Multiband feedhorns simultaneously receiving signals in plural frequency bands are known in the art. For example, see U.S. Pat. Nos. 4,910,527; 4,740,795; and 4,785,306. Multiband feedhorns are commercially available, typically comprising a single collector which collects linearly and orthogonally polarized co-located RF signals in two frequency bands. The collected multiband signals are internally separated by frequency band, and individually detected, amplified and downconverted.
Such multiband feedhorns are costly--typically tens of thousands of dollars per feedhorn--due to the different focal points and other varying characteristics of the signals in the different frequency bands.
It is desired, therefore, to provide a device which enables standard feedhorns for the C-band and Ku-band, e.g., to be used to receive signals from hybrid or co-located satellites. It is further desirable to accommodate these feedhorns utilizing to a large degree the existing mount hardwork without costly modification, yet provide a near-optimized, flexibly adjustable and low-cost multiband feedhorn mount assembly.